If you are talking about the generator running and having the battery disconnect switch in your RV in the off position; then yes you will not have any 12 volt power to your RV to operate the 12 volt lights.

If you are talking about having the battery cables disconnected physically from your house batteries and running the generator: the answer again would be yes in most cases you will not have 12 volt power to the house portion of your RV.

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